Pixels are represented by 8-bit unsigned integers, ranging in value from 0 to 255. NIH Image follows the Macintosh convention and displays zero pixels as white and those with a value of 255 as black. However, 16-bit images can be imported and scaled to 8-bits. The 16-bit to 8-bit (256 gray levels) scaling can be controlled by the user or performed automatically based on the minimum and maximum gray values in the 16-bit image. The Rescale command (in the File menu) allows the user to redo the 16-bit to 8-bit scaling at a later time. Each open image has a lookup table (LUT) associated with it. Various commands at the top of the Options menu allow the user to invert the current LUT, to specify the number of gray levels or colors it uses, or to switch to one of several built-in color tables. LUTs are automatically stored with an image when it is saved to disk, or they can also be saved separately.
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